This invention relates generally to an adhesive tape comprising a tackified natural rubber latex and a primer.
The use of natural rubber in pressure sensitive adhesives is well known. Such adhesives employ solvent-borne natural rubber, in which rubber obtained by centrifuging natural rubber latex is masticated and applied to a backing material from a solvent solution. The mastication process decreases the molecular weight of the rubber, which is believed to result in improved adhesion, possibly at the expense of cohesive strength.
There has been no significant commercial production of adhesive coatings made from water-borne rubber to date, despite the fact that environmental and health factors would favor the use of water-borne rubber. Anchorage of the adhesive to the filmic backing is a critical problem in natural rubber latex adhesive tapes. When rolls of the tape are unwound, delamination and adhesive transfer to the back side of the film remaining on the roll frequently occur. A related concern is that the tapes generate considerable noise on commercial lines when they are unwound for slitting into small rolls of adhesive tape.
Adhesive tapes from waterborne natural rubber latex have been known since the 1940""s, when methods were developed to incorporate water-insoluble solid tackifiers into aqueous dispersions. The problem of anchorage was addressed by incorporating addition polymer emulsions. Unfortunately, addition polymers are detrimental to the adhesive properties of the tape and thus are not commercially useful.
Japanese patent application, JP 11-172212 (1999), suggests that an adhesive comprising natural rubber latex, synthetic rubber latex, and an ethylene-vinyl acetate-acrylate copolymer emulsion provides good anchorage. All examples without the EVA copolymer have poor anchorage and high unwind noise due to stepped delamination. However, this reference acknowledges that use of the EVA copolymer has a detrimental effect on the adhesive properties, a deficiency which is partially overcome by the use of the synthetic rubber.
The problem addressed by this invention is to find a pressure sensitive tape adhesive containing natural rubber latex and having a good balance of adhesive properties, as well as anchorage. An additional benefit for a pressure sensitive tape would be low noise upon unwinding.
The present invention is directed to an adhesive tape comprising: (a) a polymer film; (b) a primer coat on said polymer film; and (c) an adhesive layer on the primer coat, the adhesive layer comprising: (i) a natural rubber latex; and (ii) a tackifier.
The term xe2x80x9cadhesive tapexe2x80x9d includes, for example, packaging tape, printable tape, specialty tape and protective film.
The term xe2x80x9cpolymer filmxe2x80x9d refers to any polymer in the form of a film suitable for manufacture of an adhesive tape. The preferred polymer films are those made from polyolefins, including but not limited to polypropylene, polyethylene and copolymers and combinations thereof; polyester; and polyvinyl chloride (PVC). The most preferred polymer film is oriented polypropylene film (OPP). Preferably, the polymer film has undergone corona or flame treatment to improve anchorage prior to being coated with primer and adhesive.
The term xe2x80x9cacrylic polymersxe2x80x9d refers to polymers of acrylic acid, methacrylic acid and their esters, and copolymers predominantly comprising the aforementioned monomers.
The term xe2x80x9ctackifierxe2x80x9d refers to a resin or combination of resins which imparts tack to an adhesive formulation, especially in the context of a tape adhesive. Preferred tackifiers include rosin tackifiers, polyterpene resins, aliphatic hydrocarbon resins, aromatic petroleum resins, phenol resins, styrene resins and coumarone-indene resins. A xe2x80x9crosin tackifierxe2x80x9d is a tackifier derived from a rosin, including but not limited to rosins, rosin esters, rosin derivatives, hydrogenated rosins, hydrogenated rosin esters and hydrogenated rosin derivatives. The rosin tackifier is included as the solid resin or as a liquid dispersion or emulsion. The preferred rosin tackifier is a rosin ester tackifier.
The term xe2x80x9cprimerxe2x80x9d refers to a solvent-based or aqueous material applied to a polymer film backing to form a primer coat, thereby increasing anchorage of a tape adhesive to the film. In one preferred embodiment, the primer coat comprises a natural rubber and a crosslinked polymeric phase; this primer coat typically is formed by applying to a polymer film a primer comprising: (i) a natural rubber; (ii) at least one additional polymeric component containing functional groups suitable for crosslinking; and (iii) a crosslinking agent. Preferably, at least one of the additional polymeric component(s) in the primer has a Tg no greater than xe2x88x9210xc2x0 C. In another preferred embodiment, the primer is a solvent-based or aqueous polymer composition in which the principal bulk polymeric phase has a Tg no greater than xe2x88x9210xc2x0 C. Preferably, the primer comprises an aqueous polymer composition in which at least one polymeric component has a Tg no greater than xe2x88x9210xc2x0 C., and which contains a crosslinking agent. Most preferably, the primer contains a single polymeric phase. Preferred primers include solvent-based or aqueous acrylic polymers, ethylene-vinyl acetate copolymers, ethylene-vinyl acetate-acrylic copolymers, as well as natural and synthetic rubbers such as polyisoprene, polyisobutylene, styrene-butadiene copolymers, and styrene-butadiene-acrylonitrile copolymers, and copolymers or mixtures of the aforementioned polymers. Optionally, the primer includes other additives such as plasticizers or tackifiers. In a preferred embodiment of the invention, the polymeric component of the primer comprises at least 0.2% of monomer units bearing a functional group suitable for crosslinking, e.g., carboxylic acid, hydroxyl, and amine; more preferably at least 1%. Still more preferably, the polymeric component includes an acrylic polymer having a Tg no greater than xe2x88x9210xc2x0 C. and comprising from 1% to 5% of acrylic acid or methacrylic acid monomer units, most preferably from 1.5% to 2.5%. In one preferred embodiment of the invention, the primer comprises an acrylic polymer containing at least 0.2% of monomer units having carboxylic acid functionality, a natural rubber latex, a tackifier and an isocyanate crosslinker.
Preferably, at least one crosslinking agent is added to the primer, preferably before coating the primer on the polymer film. A crosslinking agent is a material effective for crosslinking, e.g., via hydroxyl or carboxylic acid functional groups in an aqueous medium or in a solvent. Preferably, one of the crosslinking agents well known in the art is used with a primer which includes a polymeric component bearing the appropriate functional groups. These crosslinking agents include, but are not limited to, difunctional or polyfunctional isocyanates, blocked isocyanates, carbodiimides, diamines, triamines, polyamines, aziridines, amine functional resins, e.g., melamines and urea-formaldehyde resins, epoxy resins, epoxides, acids, anhydrides and polymers with conjugated unsaturation. Preferred crosslinking agents are those which are effective for crosslinking via either hydroxyl or carboxylic acid functional groups, or both, especially difunctional or polyfunctional isocyanates and carbodiimides. Preferably, the crosslinking agent is added to the primer at a level from 1% to 20% by total weight of primer, most preferably from 1% to 10% by total weight of primer.
The primer typically is coated onto the film to form a primer coat with a dry coat weight from 0.5 to 6 g/m2, preferably from 2 to 3 g/m2. The adhesive layer is applied to the primer coat. Preferably the primer coat is dried prior to application of the adhesive layer. In a preferred embodiment of the invention, the primer coat has been crosslinked by a crosslinking agent present in the primer.
Preferably, the adhesive layer contains less than 25% of a synthetic rubber. Most preferably, the adhesive layer is substantially free of synthetic rubber, e.g., polyisoprene or styrene-butadiene rubber, or other synthetic polymers.
In one embodiment of the invention, the adhesive contains from 1% to 10% of latex particles each of which has a void and a particle size from 50 to 500 nm, as described in U.S. Pat. No. 5,663,213. Such particles act as uv absorbers in the adhesive. Preferably, the adhesive contains from 3% to 6% of said latex particles.
The adhesive tape optionally contains other additives typically used in adhesive tapes, e.g., antioxidants, uv stabilizers, mechanical stabilizers, surfactants, defoamers, pigments, humectants and plasticizers.
The present invention is directed also to an adhesive tape produced by:
(a) applying a primer to a polymer film to form a primer coat; and (b) applying to the primer coat an adhesive comprising: (i) a natural rubber latex; and (ii) a tackifier. Preferably, the natural rubber latex and the tackifier are mixed to form an adhesive, which is then applied to the primer coat. Preferably, the primer coat is dried prior to application of the adhesive.